It's cheaper and easier to make square pizza boxes than round ones. The production of a round pizza box would be expensive. It would be labor intensive to assemble/construct/fold on site. It would be expensive to ship fully constructed ones, and it wouldn't be feasible for owners to store fully constructed pizza boxes. Plus not all pizza boxes are square - I've seen hexagonal and octagonal ones.

Milk cartons aren't cow shaped because the sheep consortium filed suit to block it on the grounds of that bovine supremacists were being unnecessarily insulting to sheep. "Bah!" they said... " BAH to cow shaped cartons! Vive la Feta!"... After a long carefully orchestrated fight with the COWards, as they were labelled, using subversive social indoctrination organizations like Moo-on dot org, they won their fight. And then there was the soy consortium...don't even get me started with them. If they had had their way, it'd be bean shaped.;-)

You realize that the answer you marked "Best" isn't even correct...it assumes the box would be the same size as the pizza, which is a poor design. The existing round pizza boxes (and they do exist), are all larger than the pie itself.

Of course, the irony of rewarding a pointless answer to a pointless question is almost as delicious as pizza itself.

I once lived near a pizza shop that had round boxes ( round boxes??). They were made out of that recycled looking mache type of cardboard that some egg cartons are made from. They really soaked up the grease. The lids were separate though, which made things difficult whilst doing the drunken stagger home, especially if you are anything like me and carry your pizza under your arm like an LP record.